1. Field of Invention
The present invention relates to a solar cell and, more particularly, to a tandem solar cell including an amorphous silicon carbide laminate and a multi-crystalline silicon laminate.
2. Related Prior Art
Most silicon-based solar cells are made in low-temperature processes based on plasma-enhanced chemical vapor deposition (“PECVD”). An amorphous silicon or multi-crystalline silicon layer is coated on a glass, aluminum, silicon, stainless steel or plastic substrate. A back contact is made of aluminum, gold, silver or transparent conductive oxide such as indium-tin oxide (“ITO”) and zinc oxide.
The primary advantage of the low-temperature processes is the wide variety of materials that can be used to make the substrates. However, they suffer drawbacks such as defective silicon layers, low photoelectrical conversion efficiencies and low light-soaking stability. In the PECVD, while coating the microcrystalline silicon layer, a silicon material is highly diluted in hydrogen according to the following notion:[H2]/[SiH4]>15
That is, the concentration or flow rate of H2 is more than 15 times as high as that of SiH4. The problems with the PECVD are a low growth rate of the layer, a long process and a high cost.
Regarding the making of the multi-crystalline silicon solar cells, there are various techniques such as solid phase crystallization (“SPC”) and aluminum-induced crystallization (“AIC”). The SPC is based on the PECVD. An amorphous silicon layer is deposited, intensively heated and annealed at a high temperature. Thus, a multi-crystalline silicon layer with a grain size of 1 to 2 micrometers is made.
In the AIC as shown in FIGS. 10 through 14, a substrate 51 is coated with an aluminum layer 52. An amorphous silicon layer 53 is coated on the aluminum layer 52 based on the PECVD and annealed at a temperature of about 575 degrees Celsius for a long time to form a seeding layer 54. Then, it is subjected to an epitaxial process such as the PECVD or an electron cyclotron resonance chemical deposition (“ECR-CVD”) to make a multi-crystalline silicon layer 55. The AIC however involves many steps and takes a long time. The resultant grain size is about 0.1 to 10 micrometers.
A conventional silicon-based tandem solar cell includes an upper laminate and a lower laminate. The upper laminate is an amorphous silicon p-i-n laminate. The lower laminate is a microcrystalline silicon p-i-n laminate. Thus, the infrared and visible light of the sunlit can be converted into electricity. However, the photoelectrical conversion efficiency of the conventional silicon-based tandem solar cell deteriorates quickly.
Regarding the conventional methods for making multi-crystalline silicon solar cells in the low-temperature processes based on the PECVD, there are many defects in the silicon layers, the photoelectrical conversion efficiencies are low, the light soaking stabilities low, the growth rates of the layers low, the processes long, and the costs high. Concerning the method for making multi-crystalline silicon solar cells based on the AIC, the processes are long for including many steps and therefore expensive. As for the conventional silicon-based tandem solar cell, the photoelectrical conversion efficiency deteriorates quickly.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.